Manufacturers of internal combustion engines develop engine control strategies to satisfy customer demands and meet various regulations. One such engine control strategy includes operating an engine at an air/fuel ratio that is lean of stoichiometry to improve fuel economy and reduce greenhouse gas emissions. Such engines include both compression-ignition (diesel) and lean-burn spark-ignition engines. An engine operating in a lean air/fuel ratio region may have elevated combustion temperatures, which may lead to increased oxides of nitrogen (NOx) emissions.
One known exhaust aftertreatment system and control strategy for managing and reducing NOx emissions involves injecting a reductant, such as urea, into an exhaust gas feedstream upstream of a selective catalytic reduction (SCR) device. Urea is known to decompose into ammonia (also referred to herein as NH3) when heated. Known SCR devices may store ammonia on a catalyzed surface, and NOx present in an exhaust gas feedstream may be reduced to nitrogen gas, water, and carbon dioxide by interaction with the stored ammonia in the SCR. An SCR device has a maximum ammonia storage capacity that is inversely related to its operating temperature. Known SCR catalysts operate most effectively within a temperature range between 300° C. and 500° C. When too little ammonia is stored in the SCR device as compared to NOx concentration in the exhaust feedstream, conversion efficiency may drop, and NOx emissions exiting the exhaust aftertreatment system may increase. Conversely, when the maximum ammonia storage capacity of the SCR device is exceeded, NH3 may exit the SCR device. This process is referred to as ammonia slip. Additionally, if the operating temperature of the SCR device increases rapidly at a time when ammonia storage is near its maximum, ammonia slip may also occur due to the inverse relationship between the temperature of the SCR device and its maximum ammonia storage capacity. This can occur, for example, when exhaust gas temperature increases rapidly due to heavy accelerator pedal tip-in by the vehicle operator. Known methods for managing ammonia storage in SCR devices rely on signal inputs from sensors, which attempt to measure real-time values of various engine operating parameters and exhaust gas parameters. Reductant injection may be controlled based thereon.